The present invention relates to an ATM (Asynchronous Transfer Mode) exchange system.
An ATM exchange system, which transfers all the information, including a circuit switching mode and a packet switching mode, by dividing the information into fixed length blocks (called a cell), holds promise for transmission and exchange of broadband ISDN signal (Integrated Service Digital Network).
Conventionally, a self-routing switch which uses a switch element with two inputs and two outputs and connects the same in a cascade structure has been known as an ATM exchange system.
FIG. 4 shows such a conventional exchange system. In the figure, the numerals 60-1-1 through 60-4-3 are 2.times.2 switch elements. Each of the switch elements is coupled together by junctors.
FIG. 5 shows the signal format for the exchange system of FIG. 4. The user cell 64 is attached a header (R1-R3) 61 through 63 for the routing in the switching system itself. The whole signals 61 through 64 are applied to the exchange system of FIG. 4.
The header (R1) 61 is one bit information for selecting one of two outputs of the first stage switches (60-1-1 through 60-4-1). Similarly, the header (R2) 62 and the header (R3) 63 are used for selecting one of two outputs at the second stages (60-1-2 through 60-4-2), and the third stages (60-1-3 through 60-4-3). Thus, the cell is switched by using the headers (R1 through R3) as if routing is automatic in the switching system.
However, the conventional system of FIG. 4 has the disadvantage that the delay time and the delay variation of a cell is rather large, since all the cells must be synchronous at the inputs at each of the stages, and therefore, a cell must wait in a buffer memory installed at the input of each switch element. Further, each switch element is theoretically subject to block a cell, and a cell might be lost in a switching element. When the traffic in the inputs of each of the switching elements is not uniform, the block and the loss of the cell in the switching system are increased rapidly.
A multi-point switching which distributes an input signal to more than two outputs is requested in a broadband ISDN based upon an ATM exchange system. Further, an addition and/or a modification of a connection during communication is desired. Therefore, an ATM exchange system must have an excellent switch control capability as compared with a conventional exchange system.
Further, a service class control is essential in an ATM exchange system which implies the control of the delay and the loss of a cell within an ATM exchange system. An ATM exchange system handles both real time communication and non-real time communication, and while both are important communication, there are instances when no loss of a cell is allowed and when some losses of cells are allowed. Therefore, an ATM exchange system must function as though a cell which is allowed to be lost is disregarded depending upon the congestion of output channels, and as though a cell with a small delay time is delivered to an output channel with a designated high priority.